Background: Food companies spend nearly $2 billion each year on child-directed marketing, the majority of which is used to promote high-fat energy-dense foods. Restaurant fast foods (FF) and high sugar breakfast cereals (SBC) account for the greatest advertising expenditures and exposure among young children. Because of their young age, preschool children are potentially highly susceptible to food marketing. Aims: The primary goal of this study is to determine the impact of child-directed marketing of FF and SBC on preschool children's dietary intake and BMI. Additionally, we will examine the added impact of cross- promotional tie-ins (CPTIs) and whether race/ethnicity moderates this association. Using qualitative methods, we will identify factors related to parental purchases of FF and SBC that may be modified through policy and family-level interventions. Methods: We will conduct a longitudinal survey of 800 parent-child dyads recruited from four ethnically diverse Northeast cities. The survey will include 10 assessments over a 15-month period. The baseline and final surveys will be administered in-person. Eight interim surveys will be self-administered through an iPod touch. Preschoolers' dietary intake will be assessed through repeated 7-day recalls of FF and SBC consumption and four 24-hour dietary recalls with parents as a proxy. Height and weight will be objectively measured. Children's exposure to child-directed marketing of FF and SBC will be measured through three sources: TV commercials aired on the top 6 preschool TV networks, product placements in the top box-office youth-rated movies, and onsite observations of point-of-purchase promotions at chain fast food restaurants and large supermarkets located in the four catchment areas. CPTIs will be assessed through regular monitoring of FF websites and supermarket assessments. The iPod touch surveys will be pre-programmed to ensure the survey timing corresponds to TV commercial sampling timeframes, and remotely updated to include prompts about newly released movies and CPTIs. Overall exposure will be determined by combining parent survey responses with content analyses of these three sources of food marketing. Longitudinal statistical analyses that allow for changes in dietary intake as a function of time-varying exposure to child-directed marketing will be used. Online focus groups and telephone interviews will be conducted with a purposeful sample of parents, selected based on their children's exposure to food marketing and consumption of FF and SBC. Significance: This is the first large population-based study to objectively measure multiple sources of food marketing exposure among preschoolers' and evaluate its impact on diet and BMI. Rigorous research quantifying the impact of child-directed food marketing is imperative to inform effective policy and promote responsible food marketing practices. Findings from this study will have important public health implications for preventing obesity in children.